1. Field of the Invention
The present invention relates to the field of ultrasonic scanners, and in paticular to pulse echo ultrasonic or sector scanners for use in medical diagnostics.
2. Prior Art
Ultrasonic scanning instruments are used in medical diagnostics to view regions or particular organs within the body without the necessity of surgical incission to expose the area of interest. In its most fundamental operation, the ultrasonic scanning instrument is placed in contact with surface of the body to be examined. The scanning instrument then emits a series of pulses, at an ultrasonic frequency, into the body being examined. During the time between the emissions of the pulses, the instrument searches for and detects echoes of the emitted pulses which have been reflected by the various internal objects of interest. It is these echoes and their relationship to the emitted pulses, which generates a representation or a "view" of the internal region or organs of interest.
In present day ultrasonic scanning instruments, focused ultrasound transducers are used in order to improve the resolution of the scanning instrument. Most such focused devices achieve focusing by means of a curved transducer element or by use of an acoustic lens generally attached to the front face of a planar transducer element. Of particular importance in such devices is the focal zone or linear distance in which the device is in focus. Typically, the focal zone, having a length L, of such focused devices is small, being approximately equal to L=f.sup.2 /.LAMBDA.2.lambda., where f is the focal length, .LAMBDA. is the transducer or lens effective aperture diameter and .lambda. is the wavelength of the ultrasonic wave generated by the transducer. For a typical transducer operating frequency of 2.25 MHz, .lambda.=0.068 cm, and for a typical focal length f=10 cm and .LAMBDA.=1.3 cm, the focal zone is only about 4 cm in length, whereas the extent of the zone of interest is typically 10 to 15 cm in most applications of diagnostic ultrasound.
A variety of methods may be used in an attempt to increase the focal zone of an ultrasonic scanning instrument. For example, the length of the focal zone can be increased by reduction of the transducer diameter, but this would degrade resolution and sensitivity and defeat the purpose of focusing. Also, a soft-focusing technique may be used whereby the focal zone is stretched by appropriate lens design at the expense of degraded resolution throughout the longer focal zone. Such a soft-focusing technique is disclosed in "Design of Narrowbeamwidth Transducers" by G. Kossoff in volume 35 of the Journal of the Acoustical Society of America (1963) at page 905. However, this technique represents a less than satisfactory compromise. Another possibility is to use a water bath of variable length to couple the transducer output to the patient, and then move the focal zone within the body to be examined by moving the transducer within the water path. Such a technique was disclosed by W. J. Fry, et al., in their article "Ultrasonic Visualization System Employing New Scanning and Presentation Methods" in volume 44 of the Journal of the Acoustical Society of America (1968) at page 1324. This is a relatively slow and cumbersome procedure, however, and cannot be used when rapid scanning is required, as in echocardiography or fetal scanning.
A technique which provides focusing throughout a relatively long focal zone without resolution degradation and which has recently been applied to diagnostic ultrasound devices is so called electronic focusing. This technique provides an electronically variable focus by use of an array transducer, the focus being cylindrical if a linear array is used, or spherical if an annular array is used. Such systems are described by H. E. Melton, Jr., in "Electronic Focal Scanning for Improved Resolution in Ultrasound Imaging," a Doctorate Thesis, Duke University, Durham, N. C. 1971 and by Von Ramm et al., in "Improved Resolution in Ultrasound Tomography," published in the Proceedings of the 25th Annual Conference on Engineering in Medicine and Biology (1972) at page 141.
The main drawback in electronically focused phased array systems lies in their complexity and cost. In order to provide good focusing and beam forming properties, the array systems must be made up of a relatively large number of elements. Ideally, in a uniform array, elements which are one half wavelength or less in width would be used. Since array diameters must be typically fifteen to twenty-five wavelengths in order to provide good resolution, the minimum beam width achievable being approximately f=.lambda./.LAMBDA., an array with near ideal beam characteristics should contain thirty to fifty elements. On the other hand, non-uniform arrays with fewer elements are possible at the expense of more complex delay electronics. The phased array including its delay line, switching, circuitry and amplification electronics for either array system is at present quite complex and expensive. Furthermore, in certain mechanically scanned systems, such as the ones described in a copending application by Proudian et al., Ser. No. 876,989, filed Feb. 13, 1978, it is not possible to use electronic focusing since the beam forming in these devices is achieved by rotating (acoustical) optics, not by a transducer which emits a plane wave.
Accordingly, it is a general object of the present invention to provide an improved ultrasonic focusing means.
It is a further object of the present invention to provide a simple and relatively inexpensive means to achieve dynamic focusing of an untrasound beam.
It is yet another object of the present invention to provide a means for dynamic focusing of an acoustic beam which does not require the use of a transducer array or phased array electronics.
It is still another object of the present invention to provide a means of moving the focal zone of an ultrasound diagnostic instrument within a body being examined without mechanical displacements of the transducer.
It is yet another object of the present invention to provide a means of providing variable focusing for mechanical scanners which utilize rotating transducer optics.